From Physics and Computation 2009

This variable is an array that is used for string substitutions at the end of a call to FmtPageName(). For each element in the array, the "key" (interpreted as a string) will be replaced by the corresponding "value". The variable is intended to be a place to store substitution variables that have frequently changing values (thus avoiding a rebuild of the variable cache). Also see $FmtP.

This variable is an array that is used for pattern substitutions near the beginning of a call to FmtPageName. For each element in the array, the "key" (interpreted as a pattern) will be replaced by the corresponding value evaluated for the name of the current page. This is for instance used to handle $-substitutions that depend on the pagename passed to FmtPageName(). Also see $FmtV.